1. Field of the Invention
The present invention relates to a heat treatment method of steel for improving dimensional stability, wear resistance and mechanical properties.
2. Description of the Related Art
A steel is generally subjected to a quenching to improve its hardness. Due to the quenching, the structure of the steel is transformed from austenite into martensite, to be hardened. It has also been known that the quenched steel having less retained austenite is more excellent in dimensional stability, mechanical properties and wear resistance (fatigue resistance). Hereinafter, the term xe2x80x9csteel having excellent mechanical propertiesxe2x80x9d means a steel that is less broken and cracked.
In order to further decrease the retained austenite, the quenched steel can be subsequently subjected to a tempering or sub-zero treatment.
The tempering uses the nature of the retained austenite that it is easily transformed into martensite through a high temperature treatment. Accordingly, the retained austenite starts decreasing when the steel is heated to a satisfactorily high temperature due to the tempering. For example, in case of SKH51 steel according to Japanese Industrial Standard, the retained austenite starts decreasing when the steel temperature reaches 500xc2x0 C. or higher.
However, in case that the quenched steel is tempered at too high tempering temperature, there is a problem of lowering the steel hardness, thereby decreasing the wear resistance.
Alternatively, the subzero treatment can be performed after the quenching as described above. In the subzero treatment, the quenched steel is rapidly cooled to a temperature of lower than 0xc2x0 C., also makes it possible to reduce the retained austenite in the steel, thereby giving an extremely enhanced hardness, wear resistance and dimensional stability (i.e., decreased age deformation) to the steel.
In the subzero treatment, solidified carbon dioxide (dry ice), liquid carbon dioxide (boiling point: xe2x88x9278xc2x0 C.) or liquid nitrogen (boiling point: xe2x88x92196xc2x0 C.) can be used as a cooling medium. In addition, as the subzero treatment equipment, any type can be used including types of cooling the quenched steel (i.e., xe2x80x9csteel to be treatedxe2x80x9d) by 1) immersing the steel to be treated into liquid nitrogen; 2) immersing the steel to be treated into a low temperature cooling medium such as dry ice-added ether and alcohol; 3) containing the steel to be treated in a vessel whose internal atmosphere has been cooled with a refrigerating machine; and 4) spraying liquid nitrogen or liquid carbon dioxide directly on the steel to be treated through a liquefied gas spray. The steel to be treated to the predetermined low temperature is then left at room temperature to raise the steel temperature to the ordinary temperature.
It should be noted that, in the technical field of heat treatment of steel, a high performance steel having excellent hardness, wear resistance and dimensional stability has been desired especially as materials for precise measurement and cutting tools and the like. With using such a cutting tool that is made of the high performance steel, a variety of machine parts (for example, driving members such as driving gears of automobiles and construction machines) can be manufactured.
As described above, the steel is conventionally subjected to the subzero treatment, which may be followed by the tempering, to decrease the retained austenite amount. However, the decrease is not sufficient for obtaining such a high performance steel that has excellent properties. Thus, a steel having a further decreased amount of the retained austenite has been desired. In addition, the conventional method of the subzero treatment has a problem that the steel to be treated is likely to be broken or cracked during the treatment.
To solve the problem, a heat treatment method is proposed by C. WALDMANN in ADVANCED MATERIAL and PROCESSES vol.146, No.6 (1994), p63-64. This method includes a subzero treatment in which the steel is cooled not rapidly but slowly to xe2x88x92195xc2x0 C., held for 20 to 60 hours at the temperature, then recovered to +150xc2x0 C. and slowly returned to room temperature. Another heat treatment method is proposed by P. STRATTION in METALLURGIA, vol.65, No.1(1998), p7-8. The subzero treatment of the another method includes cooling the steel slowly to xe2x88x92140xc2x0 C. at a rate of 30xc2x0 C./hr, keeping the temperature for a short time to transform the retained austenite of the steel and then recovering the steel slowly to room temperature.
According to these proposed methods, it is possible to suppress breaking and cracking of the steel. However, the retained austenite amount is not satisfactorily decreased.
Moreover, U.S. Pat. No. 5,259,200 describes a heat treatment method in which an article of steel is lowered over a liquid nitrogen bath until its temperature reaches about xe2x88x9270xc2x0 C., lowered into the bath to cool the article to about xe2x88x92196xc2x0 C., elevated out of the bath and again suspended over the bath to reach it slowly to about xe2x88x9270xc2x0 C., and allowed to heat up to room temperature.
According to this method, although the breaking and cracking can be suppressed, it is difficult to decrease the retained austenite uniformly from the surface to the deep part of the article. This may allow a large amount of the retained austenite to exist locally in the article.
The present invention has been conceived in light of these problems, and it is an object of the present invention to provide a heat treatment method of steel that is capable of transforming all of the retained austenite and extremely enhancing the steel properties such as wear resistance, mechanical properties and dimensional stability.
According to the present invention, a heat treatment method of steel includes steps of quenching a steel article, cooling the steel article at a cooling rate of 1 to 10xc2x0 C./min. to a cooling temperature, holding the steel article at the cooling temperature for a predetermined period of time and recovering the steel article to room temperature.
The cooling temperature is preferably xe2x88x92180xc2x0 C. or lower. Alternatively, it may be xe2x88x9280xc2x0 C. or lower when the heat treatment method further includes a step of tempering the steel article after recovering the steel article to room temperature.
It is preferred that the steel article is recovered to room temperature at a recovering rate of 1 to 10xc2x0 C./min.
It is also preferred that the predetermined period of time in the step of holding the steel article is one minute or more.